1. Field of the Invention
The present invention relates to an image forming method by electrophotography or electrostatic recording method. In addition, the present invention relates to a production process of a toner for developing an electrostatic latent image suited for use in the image forming method.
2. Description of the Related Art
A visualizing method of image information via an electrostatic latent image in electrophotography or the like is widely utilized now in various fields. In the electrophotography, an electrostatic latent image is formed on a photoreceptor (latent image supporting member) through charging and exposure steps, the electrostatic latent charge is developed with a developer containing a toner, and the developed image is then visualized through transfer and fixing steps. There are two types of developers used in the above-described steps, that is, a two-component developer composed of a toner and a carrier, and a one-component developer composed singly of a magnetic toner or non-magnetic toner. For the preparation of the toner, a kneading-pulverizing process which comprises melt kneading a thermoplastic resin with a pigment, a charge controller, and a release agent such as wax, cooling the kneaded mass, pulverizing it into fine particles, and then classifying the particles. If necessary, for the purpose of improving fluidity and cleaning property of the thus prepared toner, inorganic and/or organic particles may be added to the surface to the toner particles.
Although copier and printer making use of color electrophotography, and multifunction machine having a function of facsimile in addition spread widely, it is usually very difficult to use a release agent such as wax when adequate gloss and transparency for obtaining excellent OHP image are desired in the reproduction of a color image. A large amount of oil is fed to a fixing roll in order to attain good release, which however makes a copied image including OHP sticky or disturbs additional recording to an existing image by a pen. Moreover, it sometimes causes uneven gloss. Waxes ordinarily used for black-and-white copy such as polyethylene, polypropylene and paraffin damage the OHP transparency so that they are more unsuited for use.
Even though the transparency is sacrificed, it is difficult to suppress exposure of a wax on the surface when a toner prepared using the conventional kneading and pulverizing process is employed. Use of such a toner as a developer causes problems such as considerable deterioration in fluidity and filming on a developing machine and a photoreceptor.
As a fundamental method for overcoming the above-described problems, proposed is a production process of a toner by dispersing an oil phase composed of a raw material for a resin and a colorant in an aqueous phase, followed by direct polymerization, whereby the wax is included in the toner to suppress exposure thereof on the surface.
Furthermore, as a method for intentionally controlling the shape and the surface structure of a toner, production processes of a toner by the emulsion polymerization and aggregation method are proposed in JP-A-63-282752 and JP-A-6-250439. In these production processes, a toner is produced by preparing a resin particle dispersion usually by emulsion polymerization, preparing separately a colorant dispersion by dispersing a colorant in a solvent, mixing these dispersions to form aggregates having a diameter corresponding to the particle size of the toner, and then heating the aggregates to cause fusion and coalescence thereof.
These production processes not only realize inclusion of wax, but also facilitate decrease in the diameter of a toner and enable reproduction of an image with high resolution and high sharpness.
As described above, in order to provide a high quality image through the electrophotographic process and to maintain the stable performance of a toner under various kinds of mechanical stress, it is very important to select proper pigment and release agent, optimize their amounts and suppress exposure of the release agent on the surface. It is also very important to improve the gloss and releasing property in the absence of a fixing oil, and suppress a hot offset phenomenon by optimization of the characteristics of the resin.
A technology capable of fixing a toner at lower temperature is desired in order to reduce the energy consumption amount. Particularly in recent years, it is desired for thorough energy saving to stop the current passage through a fixing device when it is not used. Accordingly, the temperature of the fixing device is required to be elevated to a use temperature immediately after the current application. To this end, a heat capacity of the fixing device is preferably as small as possible. In this case, however, a difference in the temperature of the fixing device tends to increase more than as usual. This means that the overshoot of the temperature after current application increases, while the temperature drastically decreases owing to the passage of paper. Further, when a sheet of paper having a width smaller than that of the fixing device is continuously fed a difference in temperature between a paper passage portion and a non-paper-passage portion also increases. Especially, in a high-speed copier or printer, owing to shortage in a power capacity, there tends to occur such a tendency. Accordingly, there is a strong demand for the development of a toner having so-called wide fixing latitude, that is, a toner which can be fixed at low temperature and does not generate offset even in a high temperature region.
In order to decrease the fixing temperature of a toner, the use of a polycondensation type crystalline resin which exhibits a sharp melting behavior, depending on the temperature, as a binder resin constituting the toner is known. The toner using a crystalline resin tends to cause yield deformation. When the crystalline resin is used for the formation of a toner in practice, troubles such as filming on a photoreceptor due to crushing of the toner or lowering in the transfer efficiency with the passage of time cannot be avoided.
Use of a crystalline resin and a non-crystalline resin in combination is inevitable for attaining low temperature fixing property, prevention of filming on a photoreceptor and good transferring property simultaneously. Especially, a non-crystalline resin is requested to have a high performance.
When a toner is prepared by the emulsion polymerization and aggregation method as described above, it is possible to polymerize a polycondensation type resin, emulsify the polymer in a water based medium, aggregate the resulting latex together with a pigment and wax and then cause fusion and coalescence of the aggregate.
Emulsification of a polycondensation resin however requires a non-efficient and large energy consuming step such as emulsification by high shearing under heat exceeding even 150° C. or removal of a solvent after a solution having a viscosity reduced by dissolving in a solvent is dispersed in a water based medium.
In addition, it is difficult to avoid problems such as hydrolysis during the emulsification in a water based medium and occurrence of uncertain factors in material design is inevitable.
Polycondensation of a polyester resin proceeds by dehydration reaction, but an increase in molecular weight sometimes stops as the progress of the polymerization. This is presumed to occur because a viscosity of the system increases and when it reaches a predetermined value, dehydration does not occur easily. Compared with a crystalline resin which has a melting temperature and shows a drastic decrease in the resin viscosity at its melting temperature or greater, an amorphous condensation resin is highly viscous even at a temperature of Tg or greater. Reaction under severe conditions, for example, reaction for 10 hours or greater at high temperature exceeding 200° C., under stirring with a large power under high vacuum are necessary for polyester polymerization and it leads to large energy consumption. An enormous equipment investment is often required to attain durability of the reaction equipment.
Moreover, aromatic-ring-containing monomers mainly used for an amorphous polyester have low reactivity at low temperatures so that preparation of a polyester resin having a large number of rigid aromatic rings introduced in the unit thereof needs temperature conditions exceeding 150° C. An enormous equipment investment is often required to attain durability of the reaction equipment.
For example, in JP-A-2002-351140, proposed is a production process of a toner for developing an electrostatic latent image, characterized in that a raw material for toner containing at least a polyester resin is melted by heating, the melted raw material is emulsified in a water based medium to form resin fine particles, and the resin fine particles are aggregated and then fused to prepare an aggregate of the resin fine particles.
In the above-described document, a conventional polycondensation catalyst such as tetrabutyl titanate is employed as a catalyst. The monomer employed is a polycarboxylic acid such as trimellitic anhydride (TMA), a dicarboxylic acid such as terephthalic acid (TPA) and isophthalic acid (IPA), an aromatic diol such as polyoxypropylene (2,4)-2,2-bis(4-hydroxyphenyl)propane (BPA-PO) or polyoxyethylene (2.4)-2,2-bis(4-hydroxyphenyl)propane (BPA-EO) or an aliphatic diol such as ethylene glycol (EG). Reaction is effected at 220° C. for 15 hours under a normal pressure in a nitrogen gas stream, followed by gradual reduction in pressure. The reaction is then effected at 10 mmHg, whereby a polyester resin having a weight average molecular weight of from about 5,000 to 90,000 is prepared. The resulting polyester resin is melted and kneaded with a colorant and a wax. The kneaded mass MB1 is heated to 190° C. and poured in “Cavitron CD1010” (trade name; product of Eurotec) which is a dispersing and emulsifying machine. The kneaded mass is sent to Cavitron at a rate of 1 L/min while adding 0.5 wt. % dilute aqueous ammonia and heating to 160° C. by a heat exchanger. The dispersion slurry thus obtained is taken out after cooling it to 60° C. For preparing a toner from the resulting dispersion, aggregation, fusion, washing and drying steps are carried out subsequently. Such a process evidently needs an enormous energy for the preparation of the resin and emulsification of the resin and is therefore not suited for practical use.
In addition, emulsification and dispersion under such high energy conditions tend to cause decomposition of the resin. The resin therefore lacks uniformity in the composition and it is difficult to realize the uniform particle size distribution of the resin particles in the dispersion. Moreover, during storage of the dispersion, undesired aggregation of particles sometimes occurs, which also becomes a trouble for the practical use. The toner using such materials tends to cause problems not also in the initial image quality but also in the stability of the image quality during continuous printing.
There is also a report on the synthesis of a polycondensation resin in an organic solvent. For example, a production process of an unsaturated polyester resin by subjecting an aliphatic alcohol and an aliphatic polybasic acid to thermal dehydration reaction at from 100° C. to 200° C. in an organic solvent is described in JP-A-10-1536. In JP-A-8-325362, a production process of an aliphatic polyester resin which comprises reacting at least two aliphatic polyester homopolymers in an organic solvent in the presence of a catalyst is described. In JP-A-9-143253, an example of using lactic acid as a hydroxycarboxylic acid and polylactic acid as a polyhydroxycarboxylic acid is disclosed. In this document, ether solvents, halogenated hydrocarbon solvents and hydrocarbon solvents are exemplified as an organic solvent.
The technologies disclosed in the above-described documents are all related to an aliphatic polyester resin. It has been found that such resins using an aliphatic monomer are not suited for practical use at all as a resin for toner because their glass transition point is not greater than room temperature. In addition, the main object of the production process as disclosed in JP-A-9-143253 is to give biodegradability to the resin, which has no relation with the fixing property of a toner which is a technical object of the invention. In short, no suggestion useful for solving the problems of the resin for a toner is given by these disclosed technologies.
When thick paper is used as a transfer receiving material, thermal energy spent for the paper becomes large, which leads to such problems as easy change in the temperature of a fixing roll, irregular temperature distribution in the same sheet of paper or between sheets of paper and a difference in the gloss of image. Such problems become eminent at high-speed fixing and deterioration in image quality becomes a problem.
There is accordingly a demand for the development of an image forming method which can fix an image at normal temperature or by heating at low temperature, can attain uniform gloss even when thick paper specialized in a graphic art region is used, and requires reduced energy consumption.